1. Field of the Invention
The present invention relates generally to electrical circuits, and more particularly but not exclusively to discharge circuits for EMI filter capacitors.
2. Description of the Background Art
In some alternating current (AC) to direct current (DC) power supply applications, a relatively large capacitor is coupled across the AC input to serve as a noise filter for reducing the effect of electromagnetic interference (EMI) that may be generated by the AC-DC power supply. This capacitor, referred to as an “EMI filter capacitor,” is charged to high voltage by the AC signal and thus may pose a safety risk if the AC plug is disconnected from the wall socket and touched by a person. To minimize the safety risk, a resistor is typically placed in parallel with the EMI filter capacitor, such that the resistor can discharge the capacitor in a fairly short time (e.g. reduce the voltage by more than 60% in less than 1 second). This parallel resistor has the disadvantage of causing a constant power loss when the AC source is connected.
FIG. 1 shows a schematic diagram of a conventional EMI filter circuit. In the example of FIG. 1, the capacitor 104 is connected in parallel with an AC source 102, a discharge resistor 106, and input of an AC-DC power supply 108. FIG. 2 shows the voltage across the resistor 106 and the current through the resistor 106 as a function of time. The voltage across the resistor 106 is simply the sinusoidal AC voltage of the AC source 102 (waveform 202), and the current at any point in time is the voltage divided by the resistance of the resistor 106 (waveform 204). The power loss in resistor 106 is the integration of the instantaneous power loss (voltage times current) divided by the period. This constant power loss may be unacceptable in energy efficient applications.